net: hns3: Fix ping exited problem when doing lp selftest

When ping is runnig and user executes the loopback selftest, the
ping cmd will stop and exit.

This patch fixes it by using the hns3_nic_net_open/stop to offline
the netdev when doing loopback selftest.

Fixes: c39c4d98 ("net: hns3: Add mac loopback selftest support in hns3 driver")
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>